Heretofore, with regard to a polyethylene or an ethylene-.alpha.-olefin copolymer, its primary structure has been controlled by adjusting molecular weight, a molecular weight distribution or copolymerization properties (random properties, a blocking tendency and a branching degree distribution), or by adding a third component such as a diene to the polymer so as to introduce branches thereto.
On the other hand, for ethylenic polymers, various molding methods are usable, and typical known examples of the molding methods include injection molding, extrusion, blow molding, inflation, compression molding and vacuum forming. In such molding methods, the impartment of high-speed molding properties and the reduction of molding energy have been investigated for a long period of time in order to improve working properties and to thus lower a working cost, and so it is an important theme that optimum physical properties suitable for each use is imparted and the molding can be carried out with the optimum working properties.
In recent years, it has been elucidated that a uniform metallocene catalyst is excellent in the copolymerization properties between olefins, can obtain a polymer having a narrow molecular weight distribution, and has a much higher catalytic activity as compared with a conventional vanadium catalyst. Therefore, it has been expected that the metallocene catalyst will be developed in various technical fields by the utilization of such characteristics. However, a polyolefin obtained by the use of the metallocene catalyst is poor in molding and working properties, and for this reason, the application of the metallocene catalyst to the blow molding and the inflation is unavoidably limited.
In order to solve such a problem, various olefinic polymers have been disclosed into which the long-chain branches are introduced. For example, there have been disclosed (1) an olefin copolymer having the long-chain branches obtained by the use of an .alpha.,.omega.-diene or a cyclic endomethylenic diene (Japanese Patent Application Laid-open No. 34981/1972), (2) a process for preparing a copolymer containing a higher non-conjugated diene content in a high-molecular weight segment than in a low-molecular weight segment which comprises carrying out polymerization in two steps to copolymerize the non-conjugated diene with an olefin (Japanese Patent Application Laid-open No. 56412/1984), (3) an ethylene-.alpha.-olefin-1,5-hexadiene copolymer obtained by the use of a metallocene/aluminoxane catalyst (Japanese Patent Application PCT-through Laid-open No. 501555/1989), (4) a process for introducing the long-chain branches by copolymerizing an .alpha.,.omega.-diene and ethylene in the presence of a catalyst comprising a zero-valent or a divalent nickel compound and a specific aminobis(imino) compound (Japanese Patent Application Laid-open No. 261809/1990), and (5) a polyethylene containing both of the short-chain branches and the long-chain branches which can be obtained by polymerizing ethylene alone by the use of the same catalytic component as in the above-mentioned (4) (Japanese Patent Application Laid-open No. 277610/1991).
However, in the copolymer of the above-mentioned (1), a crosslinking reaction takes place simultaneously with the formation of the long-chain branches by the diene component, and at the time of the formation of a film, a gel is generated. In addition, melt properties inversely deteriorate, and a control range is extremely narrow. Moreover, there is a problem that copolymerization reactivity is low, so that low-molecular weight polymers are produced, which leads to the deterioration of physical properties inconveniently. In the preparation process of the copolymer described in the aforesaid (2), the long-chain branches are introduced into the high-molecular weight component, so that the molecular weight noticeably increases due to crosslinking, and thus insolubilization, nonfusion or gelation might inconveniently occur. Furthermore, the control range is narrow, and the copolymerization reactivity is also low, and hence, there is a problem that owing to the production of the low-molecular weight polymers, the physical properties deteriorate inconveniently. In the copolymer of the above-mentioned (3), a molecular weight distribution is narrow, and for this reason, the copolymer is unsuitable for extrusion, blow molding and film formation. In addition, since branch points are formed by the progress of the cyclizing reaction of 1,5-hexadiene, an effective monomer concentration is inconveniently low. In the process for introducing the long-chain branches described in the above-mentioned (4), there is a problem that a range for controlling the generation of a gel and the physical properties is limited. In addition, the polyethylene of the above-mentioned (5) is a polymer which contains neither ethyl branches nor butyl branches, and therefore the control of the physical properties, for example, the control of density is accomplished by methyl branches, so that the physical properties of the polyethylene tend to deteriorate.
Furthermore, there has been disclosed a method for preparing an ethylenic polymer to which working properties are imparted by the utilization of copolymerization, for example, a method which comprises forming a polymer (.eta.!=10-20 dl/g) by preliminary polymerization, and then preparing an ethylene-.alpha.-olefin copolymer by main polymerization (Japanese Patent Application Laid-open No. 55410/1992). This method has an effect that melt tension can be increased by changing the melt properties of the obtained copolymer, but it has a drawback that a film gel tends to occur.
In addition, there have been disclosed ethylenic polymers obtained in the presence of a metallocene catalyst and methods for preparing the same, for example, (1) a method for preparing an ethylenic polymer in the presence of a constrained geometrical catalyst and an ethylenic copolymer obtained by this method (Japanese Patent Application Laid-open No. 163088/1991 and WO93/08221), (2) a method for preparing a polyolefin in the presence of a metallocene catalyst containing a porous inorganic oxide (an aluminum compound) as a carrier (Japanese Patent Application Laid-open No. 100808/1992), and (3) an ethylene-.alpha.-olefin copolymer which can be derived from ethylene and the .alpha.-olefin in the presence of a specific hafnium catalyst and which has a narrow molecular weight distribution and improved melt flow properties (Japanese Patent Application Laid-open No. 276807/1990).
However, in the technique of the above-mentioned (1), the obtained ethylenic copolymer has a narrow molecular weight distribution and a narrow branching degree distribution, and both of these disadvantages cannot separately be controlled. Furthermore, there is a description that in this ethylenic copolymer, long-chain branches are present and so the ethylenic copolymer is excellent in working properties, i.e., melt flow properties, but these properties are still poor. In addition, there is no concrete description regarding other important working properties, above all, molding stabilities (a swell ratio, melt tension and the like).
According to the preparation method of the above-mentioned (2), the obtained copolymer of ethylene and the .alpha.-olefin has a large die swell ratio, but in view of the relation of the die swell ratio to the melting point of the ethylene-1-butene copolymer, it is apparent that the die swell ratio deteriorates with the rise of the melting point. Therefore, any copolymer cannot be provided in which the die swell ratio regarding a neck-in which is a trouble at the time of the formation of a film or a sheet is controllable in a wide melting point range.
On the other hand, the copolymer disclosed in the above-mentioned (3) contains an .alpha.-olefin unit as an essential unit, and it does not cover any copolymer having a resin density of more than 0.92 g/cm.sup.3. Furthermore, as in the above-mentioned (1), the obtained copolymer has a narrow molecular weight distribution and a narrow branching degree distribution, and both of these disadvantages cannot separately be controlled.
Moreover, WO94/07930 has disclosed a branched polyolefin having a straight-chain macromonomer segment as a branched component. In this technique, it has been clearly described that the activation energy of the melt flow and the melt tension increase as the effect of the long-chain branch, but there is neither any description regarding the swell ratio which is extremely important as the factor of the molding stability nor any description of a composition distribution which has a large influence on the physical properties of the polymer. In the analytical results of the macromonomers shown in examples, any description regarding an extremely important terminal unsaturated group is not present. Therefore, it is very indefinite whether or not the macromonomer is introduced into the polymer chain by the copolymerization.
On the other hand, a low-density polyethylene (LDPE) is most excellent in the working properties among presently existing polyethylenic resins, but its molecular structure is intricate and it has not all been elucidated so far. Nevertheless, it is apparent that the characteristics of the LDPE are attributed to the long-chain branch and its structure. Therefore, in order to control the swell ratio for the acquisition of the molding stability, such a mere introduction of the straight long-chain branch as disclosed in WO94/07930 is insufficient.
In the ethylenic copolymer obtained by the use of the metallocene catalyst, the molecular weight distribution is narrow and thus the branching degree distribution is also narrow as described above, so that highly branched low-molecular weight moieties are small and hence the improvement of heat-sealing properties and ESCR (environmental stress cracking resistance) can be expected. Furthermore, mechanical properties such as film impact can also be improved, but tearing strength inversely deteriorates. In addition, the ethylenic copolymer has a high uniformity, and for this reason, the transparency of the film is considered to be excellent.
On the other hand, the ethylenic copolymer obtained by the use of a conventional heterogeneous catalyst has a wide molecular weight distribution and a wide branching degree distribution. Particularly in the ethylenic copolymer, the highly branched low-molecular weight moieties are formed as by-products, and therefore the heat-sealing properties and ESCR tend to deteriorate, but the ethylenic copolymer has an advantage that the tearing strength is excellent.
As described above, the molecular weight distribution, the branching degree distribution, the long-chain branch and the structure have an extremely large influence on a resin performance, and the ethylenic copolymers in which these factors have optionally be controlled can suitably be used in various application fields.
On the other hand, when the branched ethylenic macromonomer is used as a comonomer, the long-chain branch can easily be introduced into the obtained copolymer without out gelation, and as a result, the copolymer can possess the excellent molding and working properties. Furthermore, when the branched ethylenic macromonomer is hydrogenated, the branched ethylenic polymer having a low molecular weight can be obtained which are useful as a wax in various uses such as a base oil for a lubricating oil and an additive having a controlled viscosity index.
As understood from the foregoing, the branched ethylenic macromonomer is an extremely useful compound.
As a method for preparing a low-molecular weight polyethylene (a polyethylene wax), there has been disclosed a method which comprises the gaseous phase polymerization of ethylene in the presence of hydrogen by the use of a metallocene catalyst (Japanese Patent Application PCT- through Laid-open No. 502209/1991). In this method, however, hydrogen is used for the adjustment of the molecular weight, and therefore the content of a terminal vinyl group unavoidably deteriorates (in an .alpha.-olefin/ethylene copolymer system, it further deteriorates). In consequence, the thus obtained low-molecular weight polyethylene cannot be used as the macromonomer.
Furthermore, there has also been disclosed a method for polymerizing ethylene in the presence of a Ti(OR).sub.4 (R is an alkyl group or an aryl group) catalyst (Japanese Patent Application Laid-open No. 61932/1987). However, this method intends to prepare 1-butene, which is a dimer of ethylene, in a high yield, and in this case, the dimer and the trimer of ethylene as well as an ethylene/1-butene copolymer can be produced, but an ethylene oligomer which is useful as the macromonomer cannot be produced.